Higher Dipole Bands in the NLC Accelerating
Structure

In the linac of the Next Linear Collider (NLC), trains of 95 bunches will
traverse the linac on their way to the collision point. A major concern in the linac
design is the beam emittance growth that results from the interactions of the bunches
through the long-range transverse wakefields they generate. To suppress these
wakefields, the linac accelerator structures are designed to damp and detune the
dipole modes. In this scheme, the focus is on the modes in the first dipole pass-band
since they are much stronger than those in the higher bands. However, at the level of
wakefield suppression required (about two orders of magnitude), the contribution from
the higher band modes may be significant. In this paper, we use a scattering matrix
approach to compute their contribution in a simplified 2D structure. The results are
compared with those from both an uncoupled cell calculation and a more accurate,
though more computationally intensive, finite element calculation. The effect on the
beam dynamics will also be discussed, as will experimental data.

*Work supported by the US Department of Energy contract
DE-AC03-76SF00515.